AIKEN, S.C. — Donald Trump is on his way to the Mexican border, the latest event in a presidential campaign some of his rivals would like to dismiss as a sideshow. But he’s been stealing their thunder for days and left them scrambling to adjust to a race dominated by a bombastic longshot.

The billionaire developer and reality TV host will be in Laredo, Texas, on Thursday, highlighting his unyielding stance on immigration. The trip will revisit a topic that has grown into open hostility from some Republican contenders.

From party heavyweights like Jeb Bush to recently announced candidates like Wisconsin Gov. Scott Walker and Ohio Gov. John Kasich, the contenders are confronted by Trump’s hair-trigger habit of calling out his critics by name, vilifying the party establishment and roiling the debate over immigration.

“I hope he can find the border because I’m not sure he’s ever been there before,” snapped former Texas Gov. Rick Perry.

This, after Perry denounced Trump’s campaign as a “barking carnival act.”

Sen. Lindsey Graham called Trump a “jackass” a day earlier and Trump responded by giving out the senator’s cellphone number, jamming his voice mail.

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